Ellie has a friend, Jack, that lives on a planet orbiting a star that is 20 light years away. On the night of her high school prom when Ellie is 17, she uses a radio telescope to send Jack one of her prom pictures (we will not speculate on her motives). Radio waves travel at the speed of light. Jack gets the picture and immediately uses his own radio telescope to send his own picture of his recent dinner date back to Ellie (again, let's not speculate ...). How old is Ellie when she gets Jack's picture?